REM@MC.LCS.MIT.EDU (Robert Elton Maas) (02/10/86)
U> Date: 31 Jan 86 16:05:17 GMT U> From: ucdavis!lll-crg!seismo!hao!noao!terak!mot!anasazi!will@ucbvax.berkeley. edu (Will Fuller) What a horribly long USENET path!! U> Now that we have *lots* of pretty pictures of various and U> sundry planets and satelites throughout the solar system, U> whats the latest and greatest in crator counting? Do the U> statistics clearly show a variance as a function of distance U> from the sun? Is there some sort of "hot" belt characterized U> by heavy bombardment? For the most part I think the variations you seek are swamped by varying times at which the various moons solidified and started keeping permanent record of impacts. Even icy moons like Callisto were liquid during the very early heavy bombardment, in fact the bombardment may have been the mechanism to keep them warm. The heaviest bombardment was the very early time, so a slight variation in time of solidification (a few million years one way or other) could cause a great difference in number of craters that have occurred since then (solid earlier -> early ones still there; solid later -> early ones in liquid didn't stick, only few recent ones show). It's probably not possible for a moon to be solid from day one thus keep a record of *all* impacts since day one, thus there is probably *no* unbiased count of impacts. However by factoring out gravitational purturbations from parent planet and size of moon (internal radioactive decay as well as heat retention) and other factors, it may be possible eventually go get some idea of where the meteor swarms predominated, but I would doubt it would be easy. Once we land on all the moons (with robot craft presumably; those outer moons are damn cold) and take material samples, we may be able to radioactively date the "rocks" (water-ice, frozen ammonia, etc.) like we did with the moon rocks, after which we'll able to time the individual meteors and thus get a distribution in time instead of just a gross count. At that time we'll be able to compare impact counts AT CORRESPONDING TIMES on various moons, and test your theory.
michaelm@3comvax.UUCP (Michael McNeil) (02/13/86)
In article <8602100314.AA12046@s1-b.arpa> REM%IMSSS@SU-SCORE.ARPA writes: >U> From: ucdavis!lll-crg!seismo!hao!noao!terak!mot!anasazi >U> !will@ucbvax.berkeley.edu (Will Fuller) >U> >U> Now that we have *lots* of pretty pictures of various and >U> sundry planets and satelites throughout the solar system, >U> whats the latest and greatest in crator counting? Do the >U> statistics clearly show a variance as a function of distance >U> from the sun? Is there some sort of "hot" belt characterized >U> by heavy bombardment? > >For the most part I think the variations you seek are swamped by >varying times at which the various moons solidified and started >keeping permanent record of impacts. Even icy moons like Callisto were >liquid during the very early heavy bombardment, in fact the >bombardment may have been the mechanism to keep them warm. The >heaviest bombardment was the very early time, so a slight variation in >time of solidification (a few million years one way or other) could >cause a great difference in number of craters that have occurred since >then (solid earlier -> early ones still there; solid later -> early >ones in liquid didn't stick, only few recent ones show). It's probably >not possible for a moon to be solid from day one thus keep a record of >*all* impacts since day one, thus there is probably *no* unbiased >count of impacts. However by factoring out gravitational purturbations >from parent planet and size of moon (internal radioactive decay as >well as heat retention) and other factors, it may be possible >eventually go get some idea of where the meteor swarms predominated, >but I would doubt it would be easy. I think your point about the varying dates of solidification is a good one -- probably some moons did freeze after the time of greatest bombardment. However, I believe a problem still exists with obtaining a meteoric impact time distribution even on bodies which were solid throughout that period. The reason is, on very old terrain, such as the Moon's southern hemisphere, the craters have achieved "saturation" -- that is, new impacts didn't create more craters, they just covered up existing ones. How does one find the total meteorite impact count under those circumstances? >Once we land on all the moons (with robot craft presumably; those >outer moons are damn cold) and take material samples, we may be able >to radioactively date the "rocks" (water-ice, frozen ammonia, etc.) >like we did with the moon rocks, after which we'll able to time the >individual meteors and thus get a distribution in time instead of just >a gross count. At that time we'll be able to compare impact counts AT >CORRESPONDING TIMES on various moons, and test your theory. Why are only robot craft appropriate? There is such a thing as insulation, after all. We already have to keep similarly low temperatures within and drastically higher temperatures out of the shuttle's main H2/O2 fuel tank. It's easier to insulate cold out of a ship, than keep out intense heat -- for example, if we were to land on Mercury or dip into the Sun's atmosphere. There *may* be reasons -- high radiation on certain of Jupiter's moons, for example -- why humans cannot easily explore some of the outer moons in person. But the cold per se won't stop us. -- Michael McNeil 3Com Corporation "All disclaimers including this one apply" (415) 960-9367 ..!ucbvax!hplabs!oliveb!3comvax!michaelm [Atoms] move in the void and catching each other up jostle together, and some recoil in any direction that may chance, and others become entangled with one another in various degrees according to the symmetry of their shapes and sizes and positions and order, and they remain together and thus the coming into being of composite things is effected. Simplicius, sixth century A.D.